1) Field of the Invention
The present invention relates to a probe suitable for use in an ultrasonic microscope which irradiates a sample with converging ultrasonic beam to measure and analyze acoustic characteristics of the sample on the basis of a reflected wave of the ultrasonic beam.
2) Description of the Related Art
Ultrasonic microscopes have been increasingly used in recent years as a means for determining physical properties of a surface layer of an object (sample), for example, the thickness of the surface layer, the magnitude of residual stress occurring in the vicinity of a joint interface, residual stress in a machined layer or residual stress in crystal grains, etc. These ultrasonic microscopes will be described in brief with reference to some of the accompanying drawings.
FIG. 1 is a simplified block diagram of a conventional ultrasonic microscope. In the drawing, letters X, Y and Z indicate coordinate axes. Of these, the Y-axis extends in a direction perpendicular to the drawing sheet. Designated at numeral 1 is an ultrasonic probe which is constructed of a piezoelectric element 1a and an acoustic lens lb attached to the piezoelectric element 1a. The acoustic lens lb has a semispherical lens 1c in a bottom surface thereof. There are also shown a sample 2 as an object of inspection by the ultrasonic microscope, a susceptor 3 for mounting the sample 2 thereon, a Y-axis scanning device 4 for moving the susceptor 3 in the direction of Y-axis, and an X-Y positioning device for positioning the sample 2 along X-axis and Y-axis. Numeral 6 indicates a scanning control unit, which controls the driving of the X-Y positioning device 5 and Y-axis scanning device 4 and also controls the driving of the sensor 1 in the directions of X-axis and Z-axis. Incidentally, the illustration of a drive mechanism for the sensor 1 is omitted in the drawing. Also illustrated are a liquid medium interposed between the sensor 1 and the sample 2, e.g., water, a high frequency (HF) pulse generator 9 for applying a high frequency pulsating voltage to the piezoelectric element 1a, a receiver for receiving and processing a signal from the piezoelectric element 1a, and a display 11 for performing display on the basis of a signal processed by the receiver 10.
FIG. 2 is a perspective view of the sensor 1 shown in FIG. 1. In FIG. 2, like elements similar to those shown in FIG. 1 are indicated by like reference numerals or symbols. As is apparent from the drawing, an ultrasonic wave generated at the piezoelectric element 1 propagates through the acoustic lens 1b and by the lens surface 10 in a lower portion of the acoustic lens 1b, is caused to converge into a converging ultrasonic beam B which converges at one point. The sample 2 is then irradiated by the converging ultrasonic beam B.
When a pulsating voltage is applied from the high frequency pulse generator 9 to the piezoelectric element 1a, the piezoelectric element la generates an ultrasonic wave. As is illustrated in FIGS. 1 and 2, this ultrasonic wave is caused to converge by the acoustic lens 1b and is radiated as the converging ultrasonic beam B. This ultrasonic beam B is irradiates the sample 2, and a reflected wave of the ultrasonic beam B travels backward along the radiation path and reaches the piezoelectric element 1a. Upon arrival of the reflected wave, the piezoelectric element 1a outputs an electrical signal proportional in magnitude to the reflected wave. After the receiver 10 has received, magnified and detected the electrical signal, the resultant signal is used as an intensity modulation signal so that an image of a single picture element corresponding to the electrical signal (an ultrasonic microscope image) is displayed on the display 11. By moving the sample 2 with the scanning control unit 6 and performing two-dimensional scanning with the ultrasonic beam B, a complete ultrasonic image can be obtained.
In recent years, means have been developed for investigating physical properties of a surface layer of the sample 2 (for evaluating the surface of a material) by using a converging ultrasonic beam as described above. One example of these means will hereinafter be described. When the above operation is performed while moving the sensor 1 toward the sample 2 in the direction of the Z-axis, signals are outputted from the piezoelectric element 1a, with a waveform as shown in FIG. 3. In FIG. 3, distances (Z) between the sensor 1 and the sample 2 in the direction of Z-axis are plotted along the axis of abscissas while the voltage levels (V) of signals outputted from the piezoelectric element 1a are plotted along the axis of ordinates. The distance (Z) between the sensor 1 and the sample 2 is set 0 at a certain specific position of the sensor 1, positive in the direction departing from the sample, and negative in the direction approaching the sample. The waveform shown in FIG. 3 is called a "V(Z) curve", which varies with a constant period .DELTA.Z when the sensor 1 is approaching the sample 2 beyond a certain particular distance.
Variations of the V(Z) curve with the period .DELTA.Z take place because, when the ultrasonic beam B is radiated against the sample 2, an elastic surface wave is developed in the surface layer of the sample 2 by certain components of the ultrasonic beam, said components having entered at a critical angle which is determined by the acoustic impedance of the medium (water) 7 and that of the sample 2, and a reflected wave of the elastic surface wave interferes with a reflected wave of the ultrasonic beam B. The degree of this interference successively changes depending on variations of the propagation distance of the elastic surface wave, which variations takes place as the sensor 1 is displaced in the direction of the Z-axis. The period .DELTA.Z caused by the above variations is in a certain relationship with the propagation velocity of the above elastic surface wave through the surface layer of the sample 2. The propagation velocity V.sub.R of the elastic surface wave is therefore represented by the following formula: EQU V.sub.R =V.sub.w (.DELTA.Z/.lambda.w).sup.1/2 (1)
where
V.sub.w : acoustic velocity through the liquid medium 7, and
.lambda..sub.w : wavelength of acoustic wave through the liquid medium 7.
Since the values V.sub.w,.lambda..sub.w are known in the formula (1), the propagation velocity V.sub.R of the elastic surface wave can be obtained once the period .DELTA.Z is determined from the V(Z) curve. Further, this propagation velocity V.sub.R varies depending on physical properties of the surface layer of the sample 2. It is therefore possible to determine the physical properties of the surface layer of the sample 2 on the basis of the propagation velocity V.sub.R. When the surface of the sample 2 is a machined surface, by way of example, the residual stress and thickness of the machined layer can be determined.
Incidentally, physical properties differ from one material body to another. It is desirable to permit measurement of such physical properties by an ultrasonic microscope whatever the material body is. For example, there are some material bodies whose crystalline structures are anisotropic. Even on such material bodies, predetermined measurements must be performed by an ultrasonic microscope. It is however difficult to perform such measurements using a conventional ultrasonic microscope making use of a point-converging beam. Namely, the detection of the above V(Z) curve is effected at a very small area of the sample 2. The components of the ultrasonic beam B at the radiated area are however distributed in all directions around the central axis of the beam, i.e., the beam axis. The acoustic velocity of the resulting elastic surface wave is therefore the average value of acoustic velocities in all the directions, thereby making it impossible to measure the sample 2 where the sample 2 is anisotropic.
FIG. 4 is a perspective view of a special sensor for the measurement of a sample having such anisotropy. In the drawing, there are illustrated a rectangular piezoelectric element 1a' and an acoustic lens 1b'. Since the acoustic lens 1b' is also formed in a rectangular shape, a concave surface 1c' formed in a lower part of the acoustic lens 1b' substantially defines a semi-cylindrical surface. The semi-cylindrical surface, of this sensor causes an ultrasonic beam to converge in the widthwise direction of the acoustic lens 1b'. As a consequence, the sensor can cause the ultrasonic beam to converge into a linear ultrasonic beam which extends in the direction of only one axis, i.e. in the lengthwise direction of the acoustic lens 1b'. When the sample 2 is rotated as indicated by an arrow and the propagation velocity of an elastic surface wave is measured at predetermined angular intervals, the anisotropy of the sample 2 can be determined and, further, the propagation velocity of the elastic surface wave in each of the corresponding radial directions can be measured.
The sensor depicted in FIG. 4 is however accompanied by the problem that labor and time are required for the replacement of the sensor itself. It also involves the following additional problems. This sensor has a structure permitting convergence along only one axis, whereby there is a limitation to the reduction of the length L of the acoustic lens 1b' in the longer axis thereof. Because of this limitation, their length L is generally about 2 mm. Thus, ultrasonic information available from this sensor is an average value over the 2 mm length. One of the characteristics of an ultrasonic microscope resides in that it can determine the elastic properties of a material on the basis of an extremely small area of the material. From this viewpoint, the radiation area as large as 2 mm on the sample 2 makes it absolutely impossible to exhibit the characteristic features of the ultrasonic microscope.
In addition, the surface of the sample 2 and the plane of convergence of the linear ultrasonic beam must be completely parallel to each other at the time of measurement. Formation of a slightest angle therebetween results in an error in the accuracy of measurement. the largest allowable angle tolerance is 1/100 degree. A time-consuming adjustment is usually required to control the angle not greater than the tolerance.